Is A Two Stage Furnace Worth It?

A common question when someone needs to replace their furnace is whether to choose a single stage or two stage furnace.

Most people pay very little attention to their furnace. The first time we even hear the option of two stage heating is from a contractor’s salesman. Often these salesman wax poetic about how much better a two stage furnace is over a single stage furnace.

But before you choose, I’d like to give you my real world experience with these furnaces. You’ll find it quite different from what salesmen are telling you.

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What Is A Two Stage Furnace?

Two stage heating describes a furnace with two levels of heating output. First stage (low fire) will run between 70-80% of the rated capacity of the furnace. When a furnace initially turns on it will start in this low fire mode.

The furnace will go to second stage (high fire) based on one of two conditions. 1) The time the furnace has been operating or 2) if the thermostat senses it needs to raise the temperature more than two degrees.

For example, when a furnace rated at 80,000 BTU’s first turns on it will operate at 60,000 BTU’s (75% of 80,000). After running for 10 minutes the furnace will determine it is not keeping up with the need and switch to high fire to meet demand.

The difference between single stage and two stage furnace is the single stage furnace has only one heating output. If it's rated at 80,000 BTU's, it will only operate at that capacity.

Beware Of The Two Stage Furnace Hype

Certain heating contractors will rave about the claimed benefits of the two stage furnace. "It will save you money in utilities, be quieter and keep your home more comfortable" they'll claim.

At first, the two stage furnace might look attractive. After all, who doesn’t want those things. But does a two stage furnace really offer those benefits over a single stage furnace?

Is A Two Stage Furnace More Efficient?

A two stage furnace is NOT more efficient than a single stage furnace. It will not save you any money on your utility bills compared to a single stage furnace.

Furnace manufacturers do not claim there is an efficiency gain with two stage furnaces. That idea often comes from unethical contractors trying to sell more equipment.

Efficiency is determined by heat exchanger design. Single stage and two stage furnaces of the same efficiency style use the exact same heat exchanger.

In other words, a 95% efficient furnace is 95% efficient whether its running in low or high fire. No savings.

Is A Two Stage Furnace Quieter?

Yes. But so is any new furnace compared to the old clunkers we've had in our homes for the past 30 years.

In addition, most of the noise that we complain about comes from the amount of air that is circulating through the duct work. Whether your furnace is two stage or a single stage, the required amount of air needed for the furnace to operate properly will be the same.

Does A Two Stage Furnace Keep You More Comfortable?

Who doesn’t want a more comfortable home? The two stage furnace claims it creates this by helping prevent temperature swings. How? By running at 70-80% of it’s rated capacity in first stage, the furnace will run longer thus reducing temperature swings.

The idea seems plausible but the field tested numbers do not support this claim. Here’s an example:

Let’s use an 80,000 BTU furnace as an example. If the airflow is set properly this furnace would raise the temperature of the air circulating through it about 50°. (Ex: 70° air coming into the furnace, 120° out to the supply ductwork)

At 75% capacity the furnace would be burning 60,000 BTU’s. Although heat output is changing, if the airflow is set properly the temperature of the air at 75% capacity would be only 7°-10° lower.

It would be difficult for anyone to notice that small of a change. It also ends up having little effect on the run-time of the furnace or temperature swings.

Single Stage Versus Two Stage Furnace

A single stage furnace has two major advantages over a two stage furnace. They are less prone to breakdown and cost less to repair.

The parts are also tend to be more universal. A heating contractor working on your furnace in twenty years will be more likely to find needed parts. This also helps to get your furnace up and running quickly in the event you do need a repair.

Are there any two stage furnace benefits? After digging past all the marketing it’s easy to see that a two stage furnace does not have any benefits over a single stage furnace.

Now you’re asking “If the two stage furnace doesn't save me money or make my home more comfortable why should I spend the extra money?” The answer is you shouldn’t.

Keep this in mind when deciding between a single stage or two stage furnace: A new single stage furnace installed by a good heating contractor will keep you comfortable just as well as a two stage furnace.

Why The Push For Two Stage?

Contractors often put in two stage furnaces because they’re either lazy or not very good.

Rather than taking the time to do a proper calculation to determine the correct size furnace for your home, they install an oversized furnace.

They figure the low fire mode on the two stage furnace will make up for their poor workmanship. It doesn’t and creates a host of other problems.

Manufacturers on the other hand push features that have benefit only for themselves. They’ve tried for years to improve profits by having higher equipment turnover.

They need furnaces to fail after 10-20 years not 20-30. They have succeeded by adding expensive and repair prone features all while telling you its for your benefit.

Two stage furnaces are designed to fail earlier. They won't fall apart sooner than a single stage furnace. But repairs on a two stage furnace in the 10-20 year mark will cost considerably more.

Replace the furnace or pay for high mark up parts. Either way manufacturers and unethical contractors win.

Manufacturers have done the same thing with a feature called variable speed. In fact, a two stage variable speed gas furnace is often sold as their "best" furnace.

You can avoid the manufacturers trap though. Don’t focus on equipment options. Focus on choosing the right contractor. Nothing is more important.

You will be happy with any single stage furnace as long as it's installed by a good contractor. And a good contractor won’t recommend features that you don’t need.

Thanks a lot. Your information was very helpful. After 30 years of the same oil furnace it was time to switch to a natural gas furnace. The decision was between a single or double stage furnace. Reading your article it was a know brainer. Thanks

Well thank you for this article. We have a 30 year old furnace that has really given us no problems. However, we want air conditioning and so if we are doing one we decided to do both. We have been given three bids one salesman in particular was pushing a class 2 high priced unit about 2k more than the others. He also mentioned dry air and allergies trying to upsell a new humidifier and electronic air cleaner. This article has just made our decision so much easier.

I am a home renovation contractor and until doing our research I was completely lost on this issue. Now we have go choose between Lennox and Trane or go with the cheaper model made by Coleman???? Single stage of course!

Thank you for this. I’m just putting a new system in and this was helpful I’ve wondered why many sales people and places online claim a two stage would be more fuel efficient — which seems illogical given the parameters of a fairly “closed system”.

Thank you , great article. I was wondering what size furnace I would need, mine is a 35 year old ICS 90,000 btu @ 80 % efficiency in a 2000 sq ft 1981 house with newer windows in Toronto, Ontario. It really doesn’t use a lot of gas but I believe it is near it’s end of life. I have had quotes for 80,000 and 60,000 btu @ 97%. The contractor who quoted me the 60,000 btu was sure thats all what my house needs. Neither did a load cal.

Hi Tino – I wish I was able to tell you what size furnace you need but there are just too many factors that require a physical evaluation to make that determination. What I can tell you is this; the most important aspect with regard to the size of the equipment is a performance guarantee in writing from the contractor. As an example: “Your furnace will keep your home at 70° when the outdoor temperature is at 0° or we will replace it for free.” Go with the contractor that stands behind their work in writing.

Is there any truth to what I’ve heard about a two stage furnace being better for really cold climates? I live in Chicago and need a new furnace but money is really tight and I don’t want to get sucked into features I don’t need.

Good question Patti. First I would tell you that no one NEEDS two stage equipment. Your current system is most likely single stage and if it’s sized properly kept you comfortable in the winter. Second, a home in a colder climate will not increase the reasons to buy two stage equipment. If anything it would go the other way. Two stage furnaces are marketed to help durning milder weather when you don’t need the full BTU’s of your heating system. You’re more likely to need the full output of your furnace in a colder climate. Hope this helps you save some money!

Thank you so much for this article. I have been searching in vain for objective information online about the pros and cons of single versus 2 stage gas furnaces, and yours is a lone but compelling voice of reason in the wilderness. It seems clear that the utilities, manufacturers and contractors are well aligned in promoting the “better efficiency” hype for 2 stage furnaces. It’s a real struggle for the homeowner/consumer to sort out fact from fiction, especially since 2 stage furnaces haven’t even been around long enough to assess their true lifetime/out of warranty repair costs. In my area (Pacific Northwest) our utility will not rebate for any efficiency below Energy Star level, even though a minimum 92% efficiency is already mandated by the Canadian government. HVAC quotes are structured so that the single stage is more expensive than the supposedly superior 2 stage! A single stage furnace has served my needs adequately for 30 years, and I’ve no desire to have a furnace and fan running continually in my house all day long no matter how quiet. The “better comfort” claim is purely subjective and depends entirely on the occupants’ personal heating habits and preferences, house size and layout, other heat sources, and so many other factors. If a contractor pushes the “higher efficiency” claim for 2 stage furnaces, show them this ASHRAE study whose results indicate that two-stage technology by itself does not save energy: https://aceee.org/files/proceedings/2006/data/papers/SS06_Panel1_Paper16.pdf

Good article, I’ve been researching furnaces for a little while now, a load calc of my house indicates I need a furnace with an output of approx. 63k btu’s. An 80k btu 92% furnace has an output of 73,600 btu’s 10k more than I need, and 60k btu 96% furnace has an output of 57,600 btu’s 6k less than i need. (80% AFUE furnaces are not sold here).
So the question is do I go over or under? A 2 stage 80k btu 96% offers a little bit less but more if I need it. In this case a 2 stage seems to be the answer.
What do you think? The 2 stage is only $300 more.

Glad you enjoyed the article. In answer to your question, I never recommend choosing a furnace sized lower than the calculated heat loss for your home. I also never recommend homeowners doing their own load calculations. They’re more complicated then you’d expect and free online programs that offer this service are terribly inaccurate. Make sure you use choose a contractor that can do a proper load calculation for your home. Then the sizing of the equipment is their responsibility.

Also, watch out for the seemingly low cost of $300 to “upgrade”. Manufacturers want you choosing 2 stage so down the road when it needs to be repaired more often and for a higher cost you’ll be forced to choose new equipment.